1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor thin film, and more particularly to a method for fabricating a single crystal silicon thin film at the desired location to the desired size from an amorphous or polycrystalline thin film on a substrate using laser irradiation and laser beam movement along the substrate having the semiconductor thin films being irradiated.
2. Description of the Prior Art
Generally, a thin film transistor (hereinafter, referred to as TFT), a core switching device, which is used in LCD or OLED using organic EL material, is the most important semiconductor device for the performance of a flat panel display (hereinafter, referred to as FPD).
Mobility or leakage current, a measure of the TFT performance, greatly varies depending on the state or structure of a silicon (Si) thin film, which forms an active layer, a channel for charge carrier transport. In the case of a currently commercially available LCD, the active layer of most TFTs is made of an amorphous silicon (a-Si) thin film.
Since a-Si TFT using a-Si has a very low mobility of about 0.5 cm2/Vs, there is a limitation in making all switching devices required in LCD, using a-Si TFT. This is because a switching device for a peripheral circuit of LCD needs to be operated at a very high speed, but this high speed cannot be achieved with a-Si TFT.
Thus, switching parts for the peripheral circuit, such as a driver circuit, various controllers, and a digital-analogue-converter (DAC), etc., are formed of switching devices integrated on single crystal Si to cope with a high-speed requirement for the LCD driving. On the other hand, since a-Si TFT has a switching function while showing a characteristic of low-leakage current required for ensuring image quality, it is used as a pixel-switching device.
TFT using poly-crystal Si has a high mobility of several tens to several hundreds cm2/Vs and thus can exhibit high driving speed suitable for the periphery circuit. Thus, formation of poly-Si on a glass substrate allows a pixel region and also a peripheral circuit region to be realized.
Accordingly, in the case of poly-Si TFT, separate part mounting processes required for the formation of the peripheral circuit are not required and the peripheral circuit can be formed simultaneously with a pixel region, so that a reduction in part costs for the peripheral circuit can be expected.
In addition, because of high mobility, poly-Si allows TFT to be produced at a smaller size than existing a-Si and enables the peripheral circuit and the pixel region to be formed by an integration process. Thus, making linewidth fine becomes easier so that poly-Si TFT can realize high resolution as compared to a-Si TFT-LCD.
Furthermore, poly-Si TFT can show a high-current characteristic and thus is suitable for use in OLED, a current drive type display of the next generation FDP. Thus, studies to form poly-Si and fabricate TFT on a glass substrate are actively conducted at the most recent.
In order to form poly-Si on a glass substrate, a method is typically used, in which a-Si is deposited and then crystallized into poly-Si by thermal treatment. Since the glass substrate is deformed at a higher temperature than 600° C., excimer laser annealing (hereinafter, referred to as ELA) which crystallize only a-Si without causing damage to the substrate is typically used for crystallization. Generally, upon crystallization using ELA, a-Si is irradiated with a laser so that it is melted and re-solidified to produce poly-Si. Upon crystallization, grains are randomly formed such that they have various sizes ranging from several tens nm to a few μm depending on laser irradiation conditions.
Generally, as the size of grains is increased, the mobility of a TFT device is increased and the range of parts, which can be integrated upon the integration of the peripheral circuit, becomes wider. Thus, it is preferred to obtain ELA conditions where the greatest possible size of grains can be obtained, but the greater the size of grains, the worse the uniformity of grain distribution. This causes a degradation in uniformity of device characteristics, and as a result, causes a problem in view of reliability.
Accordingly, in applying ELA-crystallized poly-Si in LCD, there is applied poly-Si having grains of a suitable size in a range where uniformity is ensured. In this case, however, poly-Si TFT having high mobility can not be fabricated due to a limitation on grain size, and thus, there is necessarily a limitation in integrating the peripheral circuit.
U.S. Pat. Nos. 6,368,945 and 6,322,625 disclose a crystallization method where large sizes of grains are obtained while ensuring uniformity. The principle of this method which is called “sequential lateral solidification” (SLS) will now be described.
FIG. 1 is a schematic view of a laser system for carrying out a SLS process. As shown in FIG. 1, a substrate 110 deposited with an a-Si film 120 is placed on a stage 100 and first irradiated with a laser beam 130 through a mask 140. In this case, there can be various patterns in the mask 140.
A typical example of this mask is a slit-shaped mask 200 as shown in FIG. 2a. In the mask 200, slits 210 having a width 220 and a length 230 are patterned. As laser beam is irradiated through the mask, the laser beam passed through the mask is irradiated in a beamlet form, and the irradiated laser beam has such energy that a-Si can be completely melted.
FIG. 3a is an enlarged view of one slit. In FIG. 3a showing a condition before laser irradiation, the reference numeral 310 represents the width of a region exposed through a slit 330, and a-Si 320 is present before exposure to laser irradiation. FIG. 3b shows a condition immediately after laser was irradiated through a slit (condition where laser was irradiated for several tens nanoseconds and then cut-off). In this case, an exposed region was melted into a liquid silicon 360, the boundary between the liquid silicon 360 and the a-Si silicon 340 is formed at the edge of the slit, and a fine poly-Si 350 is formed at the boundary. With the passage of time, the growth of grains is progressed toward the slit center, using the poly-Si 350 as a seed. In a growth process of grains, the growth of grains having slow growth rate is inhibited by grains having fast growth rate so that only some grains are continued to grow. The interface 380 between poly-Si and liquid Si-is continued to move, and ultimately, the poly-Si and the liquid Si meet with each other at the slit center as shown in FIG. 3d. In this case, the grown grain size 320 is approximately a half of the slit width. If the slit width is larger or the supercooling rate of the melted silicon after laser irradiation is fast, nucleation can occur within the liquid silicon 361 before the grains grown from both edges of the slit meet with each other at the boundary 381.
Since this circumstance is undesired, it is important that the laser irradiation conditions, the substrate temperature and the form of a slit are optimized so that the nucleation does not occur.
After the first laser irradiation was completed, a location for laser beam irradiation is shifted by a length of 450 as shown in FIG. 4a, and then, second laser irradiation is conducted through a slit. After the second laser irradiation, silicon between slit boundaries 420, 421 is converted into a liquid silicon 460, and a poly-Si region 440 which was formed after the first irradiation remains intact and is re-crystallized. In this case, at the boundary 421, a fine poly-Si region is formed, and then the growth of grains is progressed using the formed poly-Si as a seed, but at the boundary 420, the growth of grains is progressed using a region excluding the grains melted after the second irradiation among the grains formed after the first irradiation. as a seed. As a result, a structure as shown in FIG. 4c is obtained. In other words, a boundary 470 which is formed by progressing the grain growth from both sides of the slit after the second irradiation is moved by a distance 491 which was shifted from the original location for the second irradiation.
By this procedure, the grain size becomes larger due to an increase in grain length in the scanning direction. Furthermore, upon the second irradiation, since the seed crystal and a new crystal undergo continuous growth in a state where crystal orientation is not changed, a boundary 480 disappears.
FIG. 5a shows a condition after the laser beam was moved in any length while the above procedure was repeated. The lower portion of FIG. 5a shows a procedure where grains, which have been continued to grow in one direction, are present as an elongated form, and growing interfaces 520, 521 of grains, which have been grown from slit boundaries 510, 511 (, FIG. 5A ) after exposure to a slit at a front stage of growth, are grown into a liquid silicon 530.
Then, the scanning procedure is progressed to a point 551, and an a-Si region 550 is crystallized, thereby giving a structure as shown in FIG. 5b. The scanned distance is approximately equal to the reference numeral 580, and the length of the grown grains corresponds to the scanned distance 580. Since the slit is patterned according to the mask, movement of the laser beam by a given scanned distance results in formation of poly-Si patterns as shown in FIGS. 2b and 2c. The respective poly-Si patterns have a grain structure as shown in FIG. 5b. As shown in FIG. 5b, at the initial region where scanning was initiated, there is a region having many fine grains, i.e., a region shown by the reference numeral 560, since many grains competitively grow. Above the region 560, there is a region having elongated grains, i.e., a region shown by the reference numeral 570. The results of actual experiments indicate that the region 560 is smaller than 1 μm that is negligibly small in a patterned region of poly-Si (“Sequential lateral solidification of thin silicon films on SiO2”, R.S. Sposil and James S. Im, Appl. Phys. Lett., 69(19), 2864(1996)).
The SLS method is advantageous in that various shapes are obtained according to the shape of a mask, and for some masks, a single crystal Si island region can be selectively formed at a portion where a channel region of TFT is formed (U.S. Pat. No. 6,322,625).
Thus, the use of this method allows a poly-Si structure, the uniformity of device characteristics, and improvement in device performance to be obtained.
However, in a Si thin film obtained by SLS, if a regularity in the formation of a single crystal Si array having rectangular or hexagonal arrangement, and a single crystal Si island, does not coincide with the design of pixel and peripheral circuit arrangements, the uniformity of device characteristics is adversely affected.
Thus, in the existing SLS method, there can be a limitation in view of a design since a mask design for crystallization must match with pixel and peripheral circuit designs.
Furthermore, a method of making a single crystal among the SLS methods is to form a single crystal Si island in the strict sense and thus grain boundaries are present in several places of a substrate. Thus, if the pixel or the peripheral circuit is configured around the grain boundaries, excellent device characteristics and uniformity can be expected.
As a result, an ultimate solution to ensure excellent device characteristics and uniformity in any design scheme will be a method wherein single crystal silicon is formed over the entire substrate, or single crystal Si is grown only on a peripheral circuit portion and the remaining pixel region is kept at the state of a-Si so that single crystal Si having an excellent switching property for the peripheral circuit is formed outside the pixel region having low leakage current, thereby fundamentally preventing formation of the grain boundaries capable of causing non-uniformity.
For this purpose, according to the present invention, a method in which single crystal Si is easily formed at the desired location to the desired size using a simpler mask is proposed.